Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH v4 1/3] mmc: sdhci: Set DMA mask when adding host | Date | Wed, 16 Mar 2016 11:05:34 +0100 |
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On Wednesday 16 March 2016 11:26:48 Adrian Hunter wrote: > On 16/03/16 11:07, Arnd Bergmann wrote: > > On Wednesday 16 March 2016 10:43:33 Adrian Hunter wrote: > >>> + > >>> + /* 32-bit mask as default & fallback */ > >>> + if (ret) { > >>> + ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32)); > >> > >> What happens if device enumeration (e.g. of_dma_configure) has already set a > >> more restrictive DMA mask? > >> > >> > > > > In this case, dma_set_mask_and_coherent() is supposed to check the > > bus properties settings again and fail dma_set_mask_and_coherent(). > > So the logic this patch introduces will disable DMA in that case. Would it > be better just to leave the DMA mask alone (as it does now for most sdhci > drivers) in the 32-bit case?
It depends to some degree on the specific capabilities of the system.
Basically when the driver asks for a 32-bit mask, we have to check if that is actually possible, and there are a couple of possible outcomes:
- If the bus is less than 32-bit wide but the RAM is small enough to to fit within the addressable range of the bus, the dma_set_mask_and_coherent() should succeed
- If the RAM is larger than what the bus can address, but swiotlb is configured and the swiotlb bounce buffer is addressable by the bus, dma_set_mask_and_coherent() should also succeed
- If there is no swiotlb and there is RAM that fits into the 32-bit mask but that is not addressable by the bus, the dma_set_mask_and_coherent() should fail, and the driver should not use DMA.
- Similarly, if swiotlb is enabled, but its bounce buffer is not reachable by the bus, the call needs to fail and the driver must not use DMA.
Arnd
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